dcmachines 2016

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EE5301

DC Machines

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8.1 Construction

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DC Machine Construction

Figure 8.1 General arrangement of a dc machine

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DC Machines• The stator of the dc motor has

poles, which are excited by dc current to produce magnetic fields.

• In the neutral zone, in the middle between the poles, commutating poles are placed to reduce sparking of the commutator. The commutating poles are supplied by dc current.

• Compensating windings are mounted on the main poles. These short-circuited windings damp rotor oscillations. .

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DC Machines• The poles are mounted on an

iron core that provides a closed magnetic circuit.

• The motor housing supports the iron core, the brushes and the bearings.

• The rotor has a ring-shaped laminated iron core with slots.

• Coils with several turns are placed in the slots. The distance between the two legs of the coil is about 180 electric degrees.

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DC Machines• The coils are connected in series

through the commutator segments.

• The ends of each coil are connected to a commutator segment.

• The commutator consists of insulated copper segments mounted on an insulated tube.

• Two brushes are pressed to the commutator to permit current flow.

• The brushes are placed in the neutral zone, where the magnetic field is close to zero, to reduce arcing.

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DC Machines• The rotor has a ring-shaped

laminated iron core with slots.

• The commutator consists of insulated copper segments mounted on an insulated tube.

• Two brushes are pressed to the commutator to permit current flow.

• The brushes are placed in the neutral zone, where the magnetic field is close to zero, to reduce arcing.

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DC Machines• The commutator switches the

current from one rotor coil to the adjacent coil,

• The switching requires the interruption of the coil current.

• The sudden interruption of an inductive current generates high voltages .

• The high voltage produces flashover and arcing between the commutator segment and the brush.

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|

Shaft

Brush

Coppersegment

InsulationRotor

Winding

N S

Ir_dcIr_dc/2

RotationIr_dc/2

Ir_dc

12

3

45

6

7

8

Polewinding

Figure 8.2 Commutator with the rotor coils connections.

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Figure 8.3 Details of the commutator of a dc motor.

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Figure 8.4 DC motor stator with poles visible.

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Figure 8.5 Rotor of a dc motor.

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Figure 8.6 Cutaway view of a dc motor.

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8.2.1 DC Motor Operation

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8.1 DC Motor • The direct current (dc) machine can be used as

a motor or as a generator. • DC Machine is most often used for a motor. • The major advantages of dc machines are the

easy speed and torque regulation. • However, their application is limited to mills,

mines and trains. As examples, trolleys and underground subway cars may use dc motors.

• In the past, automobiles were equipped with dc dynamos to charge their batteries.

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8.1 DC Motor • Even today the starter is a series dc motor • However, the recent development of power

electronics has reduced the use of dc motors and generators.

• The electronically controlled ac drives are gradually replacing the dc motor drives in factories.

• Nevertheless, a large number of dc motors are still used by industry and several thousand are sold annually.

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DC Motor Operation• In a dc motor, the stator

poles are supplied by dc excitation current, which produces a dc magnetic field.

• The rotor is supplied by dc current through the brushes, commutator and coils.

• The interaction of the magnetic field and rotor current generates a force that drives the motor

|

Shaft

Brush

Coppersegment

InsulationRotor

Winding

N S

Ir_dcIr_dc/2

RotationIr_dc/2

Ir_dc

12

3

45

6

7

8

Polewinding

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• The magnetic field lines enter into the rotor from the north pole (N) and exit toward the south pole (S).

• The poles generate a magnetic field that is perpendicular to the current carrying conductors.

• The interaction between the field and the current produces a Lorentz force,

• The force is perpendicular to both the magnetic field and conductor

(a) Rotor current flow from segment 1 to 2 (slot a to b)

Vdc30 NS

Bv

v

a

b

1

2

Ir_dc

(b) Rotor current flow from segment 2 to 1 (slot b to a)

30 NS Vdc

a

b

1

2

B

v v

Ir_dc

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• The generated force turns the rotor until the coil reaches the neutral point between the poles.

• At this point, the magnetic field becomes practically zero together with the force.

• However, inertia drives the motor beyond the neutral zone where the direction of the magnetic field reverses.

• To avoid the reversal of the force direction, the commutator changes the current direction, which maintains the counterclockwise rotation. .


Vdc30 NS

Bv

v

a

b

1

2

Ir_dc


30 NS Vdc

a

b

1

2

B

v v

Ir_dc

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8.2.1 DC Motor Operation• Before reaching the neutral zone,

the current enters in segment 1 and exits from segment 2,

• Therefore, current enters the coil end at slot a and exits from slot b during this stage.

• After passing the neutral zone, the current enters segment 2 and exits from segment 1,

• This reverses the current direction through the rotor coil, when the coil passes the neutral zone.

• The result of this current reversal is the maintenance of the rotation.


Vdc30 NS

Bv

v

a

b

1

2

Ir_dc


30 NS Vdc

a

b

1

2

B

v v

Ir_dc

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8.2.2 DC Generator Operation

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• The N-S poles produce a dc magnetic field and the rotor coil turns in this field.

• A turbine or other machine drives the rotor.

• The conductors in the slots cut the magnetic flux lines, which induce voltage in the rotor coils.

• The coil has two sides: one is placed in slot a, the other in slot b.

30 NS Vdc

Bv

v

a

b

Ir_dc


30 NS Vdc

a

b

1

2vv

B

Ir_dc


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• In Figure 8.11A, the conductors in slot a are cutting the field lines entering into the rotor from the north pole,

• The conductors in slot b are cutting the field lines exiting from the rotor to the south pole.

• The cutting of the field lines generates voltage in the conductors.

• The voltages generated in the two sides of the coil are added.

30 NS Vdc

Bv

v

a

b

Ir_dc


30 NS Vdc

a

b

1

2vv

B

Ir_dc


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• The induced voltage is connected to the generator terminals through the commutator and brushes.

• In Figure 8.11A, the induced voltage in b is positive, and in a is negative.

• The positive terminal is connected to commutator segment 2 and to the conductors in slot b.

• The negative terminal is connected to segment 1 and to the conductors in slot a.

30 NS Vdc

Bv

v

a

b

Ir_dc


30 NS Vdc

a

b

1

2vv

B

Ir_dc


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• When the coil passes the neutral zone: – Conductors in slot a are

then moving toward the south pole and cut flux lines exiting from the rotor

– Conductors in slot b cut the flux lines entering the in slot b.

• This changes the polarity of the induced voltage in the coil.

• The voltage induced in a is now positive, and in b is negative.

30 NS Vdc

Bv

v

a

b

Ir_dc


30 NS Vdc

a

b

1

2vv

B

Ir_dc


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• The simultaneously the commutator reverses its terminals, which assures that the output voltage (Vdc) polarity is unchanged.

• In Figure 8.11B – the positive terminal is

connected to commutator segment 1 and to the conductors in slot a.

– The negative terminal is connected to segment 2 and to the conductors in slot b.

30 NS Vdc

Bv

v

a

b

Ir_dc


30 NS Vdc

a

b

1

2vv

B

Ir_dc


•Where, P = no of poles φ = flux per pole Z= No. of conductors A = No. of parallel paths and N is the speed of the DC Motor.

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The back emf like in case of a generator is represented by

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8.2.3 DC Machine Equivalent Circuit

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Generator

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8.2.3 DC Generator Equivalent circuit

• The magnetic field produced by the stator poles induces a voltage in the rotor (or armature) coils when the generator is rotated.

• This induced voltage is represented by a voltage source. • The stator coil has resistance, which is connected in series.• The pole flux is produced by the DC excitation/field

current, which is magnetically coupled to the rotor• The field circuit has resistance and a source• The voltage drop on the brushes represented by a battery

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• Figure 8.12 Equivalent circuit of a separately excited dc generator.

RfRa

Vbrush

VdcEagVf

max

IfIag

Load

Mechanicalpower in

Electricalpower out

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• The magnetic field produced by the stator poles induces a voltage in the rotor (or armature) coils when the generator is rotated.

• The dc field current of the poles generates a magnetic flux

• The flux is proportional with the field current if the iron core is not saturated:

fag IK1

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• The rotor conductors cut the field lines that generate voltage in the coils.

• The motor speed and flux equations are :

vBNE grag 2

2gDv ggag DB

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• The combination of the three equation results the induced voltage equation:

• The equation is simplified.

agrggrg

grgrag NDBND

BNvBNE

222

fmfragrag IKIKNNE 1

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• When the generator is loaded, the load current produces a voltage drop on the rotor winding resistance.

• In addition, there is a more or less constant 1–3 V voltage drop on the brushes.

• These two voltage drops reduce the terminal voltage of the generator. The terminal voltage is;

brushaagdcag VRIVE

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Motor

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8.2.3 DC Motor Equivalent circuit

• Figure 8.13 Equivalent circuit of a separately excited dc motor• Equivalent circuit is similar to the generator only the current directions are different

RfRa

Vbrush

VdcEamVf

max

IfIam

Mechanicalpower out

Electricalpower in

DC Powersupply

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8.2.3 DC Motor Equivalent circuit• The operation equations are:• Armature voltage equation

brushaamamdc VRIEV

The induced voltage and motor speed vs angular frequency

fmam IKE mn 2

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8.2.3 DC Motor Equivalent circuit• The operation equations are:• The combination of the equations results in

The current is calculated from this equation. The output power and torque are:

mamdcamfm RIVEIK

amamout IEP fammout IIKP

T

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8.2.4 DC Machine Excitation Methods

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DC Motor Operation

• There are four different methods for supplying the dc current to the motor or generator poles:

– Separate excitation;– Shunt connection– Series connection– Compound

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• Figure 8.14 Equivalent circuit of a shunt dc motor

DC Powersupply

Vdc

Eam

Iam

max Rf

If

Ra

Vbrush

Im

Pout

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• Figure 8.15 Equivalent circuit of a series dc motor

Vdc

Eam

Rf

RaVbrush

Im

max

DC Powersupply

Pout

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• Figure 8.16 Equivalent circuit of a compound dc motor

Vdc

Eam

Rfs

RaVbrush

Im

max

DC Powersupply

Rfp

Iam

IfpPout

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